Maintaining Prostate IMRT Plan Quality Using the Fraction of Rectum Overlapping the PTV

Purpose: It is clinically meaningful to generate the optimal intensity-modulated radiation therapy (IMRT) treatment plans within the restricted time with equally good quality for different patients, which is planner-dependent. We developed a planner-independent method for a clinical decision of prostate IMRT plan using the rectal complication probability with the volume of rectum overlapping the PTV.

Methods:Eighteen prostate IMRT plans were retrospectively analyzed. The prescription dose in the all the planning target volumes (PTVs) was normalized with 76 Gy in 2 Gy fractions. We found a correlation between the fraction of rectum overlapping the PTV and the rectal normal tissue complication probability (NTCP), which is the predictable NTCP curve, which means the lower bound of the rectal NTCP. The plans beyond the predictable NTCP criteria were generated again with seven 10 MV beams from the Varian Clinac iX linear accelerator. The beams were arranged coplanar at: 0, 50, 100, 140, 220, 260, and 310. IMRT plans were generated using direct machine parameter optimization in the Pinnacle3.

Results:All the regenerated plans came to be included in the predictable NTCP criteria. In our patient data, two plans were re-planned: the NTCP for patient 7 was changed from 10.9 to 8.8 and the other NTCP for patient 14 was from 15.6 to 12.9. The rectal NTCP is clinically more meaningful than the mean dose of rectum.

Conclusion:This model can predict the rectum NTCP before IMRT planning, to control the IMRT plan quality during planning. Hence we can maintain prostate IMRT plan quality using the fraction of rectum overlapping the PTV.